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As many as 145 pilot whales have died after a mass stranding in Mason Bay on Stewart Island.

A hiker reportedly spotted the whales on Saturday and notified Department of Conservation staff at 10.30pm.

There were two pods stranded at the southern end of Mason Bay, approximately 2km apart.

DoC Rakiura operations manager Ren Leppens said half of the whales had already died by the time they were found and due to the condition of the remaining whales and the remote, difficult to access location, the decision was made to euthanise the remainder.

"Sadly, the likelihood of being able to successfully refloat the remaining whales was extremely low. The remote location, lack of nearby personnel and the whales' deteriorating condition meant the most humane thing to do was to euthanise."

"However, it's always a heart-breaking decision to make."

Marine mammal strandings are a relatively common occurrence on New Zealand shores, with DoC responding to an average 85 incidents a year – mostly of single animals.

On Sunday 10 pygmy killer whales stranded at Ninety Mile Beach. Two have since died and attempts were being made to refloat the others.

It was the second stranding in the Far North in three days after a 15m sperm whale died on Tokeroa Beach on Saturday, after washing ashore on Friday.